Add repo-sync action to pull updates from the public repo

See https://github.com/repo-sync/repo-sync

.github/workflows/repo-sync.yml

@@ -0,0 +1,25 @@
+name: Repo Sync
+
+on:
+  workflow_dispatch:     # allows triggering this manually through the Actions UI
+
+jobs:
+  repo-sync:
+    name: Repo Sync
+    environment: repo-sync
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: repo-sync/github-sync@v2
+      name: Sync repo to branch
+      with:
+        source_repo: ${{ secrets.SOURCE_REPO }}
+        source_branch: master
+        destination_branch: ${{ secrets.INTERMEDIATE_BRANCH }}
+        github_token: ${{ secrets.GITHUB_TOKEN }}
+    - uses: repo-sync/pull-request@v2
+      name: Create pull request
+      with:
+        source_branch: ${{ secrets.INTERMEDIATE_BRANCH }}
+        destination_branch: master
+        github_token: ${{ secrets.GITHUB_TOKEN }}